Generally, slot array antennas, in which two or more slots which resonate with transceiving electromagnetic waves are arranged in a side face of a waveguide, have high side lobe level characteristics with a low gain. A slot array antenna where the characteristics are improved is proposed in WO2008/018481.
FIGS. 18A and 18B show an example of the slot array antenna disclosed in WO2008/018481. FIG. 18A is an elevational view showing a configuration of the slot array antenna disclosed in WO2008/018481, and FIG. 18B is a bottom view. A slot array antenna 100 shown in FIGS. 18A and 18B mainly includes an emission waveguide 130 where two or more slots 131 are arranged two-dimensionally and an introduction waveguide 120 where two or more slots 121 for introducing electromagnetic waves into a waveguide space of the emission waveguide 130 are arranged.
The emission waveguide 130 is a waveguide having a rectangular cross section, where many rows of slots 131 are arranged parallel to each other in the x-axis direction in a first conductor plane metal plate 130a of the emission waveguide 130, while each row of the slots being arranged in line in the y-axis direction. A second conductor plane metal plate 130b is also arranged in parallel to the first conductor plane metal plate 130a. The first and second metal plates 130a and 130b for conductor planes are joined with each other with screws 133 and supported by support members 132 which are disposed at location corresponding to nodes of the electromagnetic waves propagating within the emission waveguide 130 and nodes of tube wall current. The waveguide space is formed between the first conductor plane metal plate 130a and the second conductor plane metal plate 130b. A radio wave absorber 134 is provided at one end of the emission waveguide 130 in the x-axis direction, and a short circuit surface is formed at the other end. Two side faces which connect the first conductor plane metal plate 130a and the second conductor plane metal plate 130b (top and bottom surfaces) also serve as short circuit surfaces.
The introduction waveguide 120 is formed so as to have the second conductor plane metal plate 130b of the emission waveguide 130 as one side thereof. The introduction waveguide 120 is formed by attaching a gutter-shaped metal plate to the second conductor plane metal plate 130b. The introduction waveguide 120 is attached so that the y-axis direction is oriented along a propagating direction of the electromagnetic waves. The slots 121 of the introduction waveguide 120 are formed in the second conductor plane metal plate 130b and arrayed in the y-axis direction.
The slots 121 are arranged along the propagating direction of the electromagnetic waves within the introduction waveguide 120 at every half wavelength or at every positive number times of the half wavelength of the electromagnetic waves within the introduction waveguide 120. The electromagnetic waves in a high order mode in which two or more magnetic field loops are located in line in the propagating direction of the electromagnetic waves within the introduction waveguide 120 are excited by the emission waveguide 130. The slots 131 are formed such that primary polarized wave planes of emission electric fields caused by the slots 131 couple to the electromagnetic waves in the high order mode face to be directed in the same direction, and other polarized wave components which intersect perpendicular to the primary polarized wave planes are cancelled with each other.
The slot array antenna 100 constituted as described above can emit the electromagnetic waves from the first conductor plane metal plate 130a, while interrupting the tube wall current in the high order mode at suitable locations by the slots 131 formed in the emission waveguide 130. For this reason, although it is a slot array antenna, a side lobe control is easy. For example, the geometry of the slots 131 of the emission waveguide 130 are set so that the intensities of the electromagnetic waves to be emitted become lower as the slots are separated from the center of the emission waveguide 130 in the electromagnetic wave propagating direction toward both ends of the emission waveguide 130. Thereby, the side lobes of the electromagnetic waves emitted from the emission waveguide 130 are effectively suppressed.
Again, although the slot array antenna 100 disclosed in WO2008/018481 is a slot array antenna, it has the outstanding characteristic that the side lobe control is easy.
However, in a case where the slot array antenna 100 disclosed in WO2008/018481 is used as an antenna for ship radar, when the ship presents surge and sway motions and an angle of the antenna with respect to the vertical axis (an angle with respect to the z-axis in the y-z plane) varies, a situation where the suppressing effect of the side lobes is reduced may occur.